The lysosomal storage diseases are inherited metabolic deficiencies that produce fatal degenerative syndromes, including severe mental retardation. Previous data indicate that supplying the normal gene product during fetal life may significantly reduce pathology compared to postnatal treatment. The applicant will evaluate methods to transfer genes into the developing fetus to attempt to reduce the severity, or even forestall the development, of pathology. Animals with beta- glucuronidase (GUSB)- deficient mucopolysaccharidosis (MPS) type VII (Sly disease) will be used as a model system. The applicant has developed retroviral vectors to transfer and express high levels of GUSB in MPS VII cells and shown that partial reversal of the disease can be achieved by hematopoietic stem cell-directed gene transfer in adults. However, the blood-brain barrier prevents therapeutically effective entry of GUSB from normal bone marrow or retrovirus vector-corrected hematopoietic cells when transplanted postnatally. The disease in the brain can be permanently corrected of normal enzyme is delivered to the brain side of the blood-brain barrier by using neural progenitor cells, which indicates that it may be necessary to use separate treatment strategies to correct both the brain disease and the visceral/skeletal disease in patients. The investigator will therefore transfer the gene into pluripotent hematopoietic stem cells in the fetal liver and into multi-lineage neural progenitor cells in the fetal brain. An normal GUSB cDNA will be transferred into MPS VII fetuses by infecting fetal liver hematopoietic stem cells and multipotent neural progenitor cells ex vivo and transplanting them into the fetus in utero. The fate of the engineered cells will be determined using the vector proviruses and genetic markers, expression of the transferred GUSB will be determined by enzymatic activity in situ and by quantitative assays, and the long-term effects on pathology and safety of the treatments will be evaluated. These studies will determine the extent of reconstitution that can occur by vector-infected hematopoietic stem cells after fetal transplantation, especially whether the progeny cells can effectively colonize the brain if transplanted in utero, and will determine if neural progenitor cells can be engineered to give rise to GUSB-producing cells within the central nervous system.